Recently, a lithium secondary battery is generally employed as an electric source for driving small electronic devices. The lithium secondary battery mainly comprises a positive electrode, a non-aqueous electrolytic solution, and a negative electrode. The non-aqueous lithium secondary battery preferably employs a positive electrode of lithium complex oxide such as LiCoO2 and a negative electrode of carbonaceous material or lithium metal. The non-aqueous electrolytic solution for the lithium secondary battery preferably employs carbonate compounds such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (MEC).
Nevertheless, it is desired to provide a secondary battery having more excellent performances in the cycle performance and electric capacity.
In the lithium secondary battery using a positive electrode of LiCoO2, LiMn2O4, or LiNiO2, oxidative decomposition of a portion of a solvent of the non-aqueous electrolytic solution takes place in the electric charging stage. The decomposition product disturbs electrochemical reaction of the battery so that the battery performance lowers. It is considered that the oxidative decomposition occurs in the solvent on the interface between the positive electrode and the non-aqueous electrolytic solution. Therefore, the battery performances such as cycle performance and electric capacity are not satisfactory for batteries to be repeatedly charged and discharged at a maximum working voltage exceeding 4.1 V.
Moreover, in the lithium secondary battery using particularly a negative electrode of carbonaceous material of high crystallinity such as natural graphite or artificial graphite, reductive decomposition of the solvent of the non-aqueous electrolytic solution takes place on the surface of the negative electrode in the charging stage. The reductive decomposition on the negative electrode takes place after repeated charging-discharging procedures even in the case of using EC which is generally employed in the non-aqueous electrolytic solution.
Accordingly, the battery performances such as cycle performance and electric capacity are at present considered to be not satisfactory.
When the lithium secondary battery is so overcharged as exceeding an ordinary working voltage, an excessive amount of lithium is released from the positive electrode, while an excessive amount of lithium produces dendrite on the negative electrode. Therefore, both of the positive electrode and negative electrode become chemically unstable. When both of the positive electrode and negative electrode become chemically unstable, the carbonate in the non-aqueous electrolytic solution rapidly decomposes under exothermic reaction. The rapid exothermic reaction causes abnormal heat production of the battery and impairs safety of the battery. These problems become more serious, as the energy density of the lithium secondary battery increases. However, at present, the safety in keeping the battery from overcharging and the battery performances such as cycle performance, electric capacity and storage endurance are still not satisfactory.
The present invention has an object to provide a lithium secondary battery that is so free from the above-mentioned problems as to be excellent in the safety in overcharging and further in battery performances such as cycle performance, electric capacity and storage endurance under the charged conditions, and to provide a novel non-aqueous electrolytic solution favorably employable for the preparation of the lithium secondary battery.